This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 9-194163, filed on Jul. 18, 1997, No. 9-199973, filed on Jul. 25, 1997, No. 9-277613 filed on Oct. 9, 1997, and No. 10-147939 filed on May 28, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a semiconductor substrate having a semiconductor layer for device formation provided on a base substrate in such a state that the semiconductor layer is electrically insulated from the base substrate, and to a method for manufacturing the semiconductor substrate.
2. Description of the Related Art
A semiconductor substrate of this kind includes for example an SOI (Silicon On Insulator) substrate wherein an oxide film for insulation is formed on a silicon substrate and a single crystal (monocrystalline) silicon film is formed on the oxide film as a semiconductor layer. When this kind of semiconductor substrate is used, it is unnecessary to separately conduct a step for isolating the semiconductor layer from the substrate, the isolation performance is improved, and it is possible to form devices in the single crystal silicon film at a high integrated density to make an integrated circuit.
As a method for manufacturing the single crystal silicon film of the SOI structure, various methods have been proposed. For example JP-A-5-211128 discloses a semiconductor thin film manufacturing method wherein the single crystal silicon film is manufactured in three steps. This manufacturing method will now be described with reference to FIGS. 1A-1D.
In a first step, as shown in FIG. 1A, ions of hydrogen gas or noble gas are implanted into a semiconductor substrate material 1 through a contamination protective film 1a formed on an upper face of the semiconductor substrate material 1. An ion-implanted region 2 in which implanted ions are distributed is thus formed at a predetermined depth in the semiconductor substrate material 1. In a second step, as shown in FIG. 1B, a base substrate 3 made of at least one rigid material is joined to the ion implantation side face of the semiconductor substrate material 1 by bonding or the like. Here, a substrate made of a semiconductor material can be used as the base substrate 3, and to eventually form an SOI substrate, preferably an insulating film 4 such as an oxide film is formed on the base substrate 3 before the base substrate 3 is joined to the semiconductor substrate material 1.
As a third step, heat treatment is carried out on the semiconductor substrate material 1 and the base substrate 3 thus joined together, so that a thin film part is detached from the semiconductor substrate material 1. The boundary of the detachment is a micro-void (minute bubbles) part formed in the ion-implanted region 2. Accordingly, as shown in FIG. 1C, an SOI substrate 6 consisting of a single crystal silicon film 5 bonded to the base substrate 3 with the insulating film 4 therebetween is formed. In practice, because the detachment face of the single crystal silicon layer 5 has surface steps of several nm in size and a defective layer thereon, the detachment face is polished or etched to be flattened. At the same time, the thickness of the single crystal silicon film 5 is controlled to be a predetermined thickness (for example 0.1 xcexcm), thereby finishing the SOI substrate 6 as shown FIG. 1D.
However, when the SOI substrate 6 is manufactured by the steps described above, in practice the following problem may arise. That is, as shown in FIG. 2, in the heat treatment of the third step mentioned above, when the detachment phenomenon at the above-mentioned micro-void part occurs, the surface portion of the semiconductor substrate material 1, which is supposed to form the single crystal silicon film 5 bonded to the base substrate 3, is liable to partially remain on the semiconductor substrate material 1. In FIG. 2, the remaining parts are shown with letter Q. As a result, an unevenly detached silicon layer is bonded to the base substrate 3, resulting in a significant reduction in the quality and deterioration of yield of the SOI substrate 6. Although because of drawing restrictions the phenomenon of the remaining parts Q arising is schematically shown in FIG. 2, the remaining parts are actually produced in many locations distributed over the entire area of the semiconductor substrate material 1.
It can be supposed that this kind of phenomenon occurs because the bonding of the semiconductor substrate material 1 to the base substrate 3 is not effected uniformly over the entire region of the bonding. Such bonding defects may be caused, for example, by flatness of the contamination protective film 1a on the semiconductor substrate material 1 being degraded by the ion implanting step, or by contaminants remaining in the vicinity of the surface of the contamination protective film 1a in a segregated state. When such bonding defects exist, even when the kind of phenomenon shown in FIG. 2 does not occur, the bonding strength between the base substrate 3 and the single crystal silicon flim 5 may be insufficient, and this can cause the insufficient reliability of the SOI substrate 6.
Further, in the above mentioned method, the semiconductor substrate material 1 and the base substrate 3 are made of the same material so as to prevent warps from occurring to the bonded semiconductor substrate material 1 and the base substrate 3 bonded together due to a difference in thermal expansion coefficient therebetween. However, in this case, it is necessary for both of the semiconductor substrate material 1 and the base substrate 3 to be made from a high quality single crystal silicon, resulting in increase in cost. Additionally, the method, especially the third step, necessitates relatively long time, and this also results in increase in cost.
The present invention is made in view of the above problems. A first object of the present invention is to provide a semiconductor substrate including a semiconductor layer bonded to a base substrate in an insulated state from the base substrate with a sufficient bonding strength and with good yield. A second object of the present invention is to provide a method for manufacturing a high-quality semiconductor substrate including a semiconductor layer bonded to a base substrate in an insulated state from the base substrate at low cost.
To achieve a first object of the present invention, a semiconductor substrate of the present invention is manufactured such that a semiconductor layer is bonded to a base substrate through a natural oxide layer. Accordingly, the bonding strength between the base substrate and the semiconductor layer increases. Alternatively or additionally, before the base substrate is bonded to a semiconductor substrate material and before a ion-implanted layer is formed in the semiconductor substrate material, a contamination protective layer may be formed on the semiconductor substrate material. In this case, ions are implanted into the semiconductor substrate material through the contamination protective layer to form the ion-implanted layer, and at least a part of the contamination protective layer is removed before the base substrate and the semiconductor substrate material are bonded to one another. Accordingly, even when the flatness of the contamination protective layer is degraded by ion-implantation or even when contaminants remain in the vicinity of the surface region of the contamination protective layer, the bonding state can be made uniform over the entire bonding area. Therefore, the semiconductor layer can be detached from the semiconductor substrate material along with the base substrate without partially remaining on the semiconductor substrate material, so that the yield of the semiconductor substrate is improved.
Preferably, before the base substrate and the semiconductor substrate material are detached from one another at the ion-implanted layer by a heat treatment, another heat treatment is carried out at a temperature lower than that of the heat treatment for detaching not to cause the detachment of the base substrate and the semiconductor substrate material. More preferably, after the heat treatment for detaching is carried out, another heat treatment is carried out at a temperature higher than that of the heat treatment for detachment. Accordingly, the bonding strength between the base substrate and the semiconductor layer is further increased.
To achieve a second object of the present invention, a semiconductor substrate of the present invention includes a base substrate that holds a monoclystalline semiconductor layer thereon via an insulating layer and has a quality inferior to that of the single crystal semiconductor layer. Accordingly, the semiconductor substrate can be manufactured at low cost. The semiconductor substrate can be manufactured by bonding the base substrate to a semiconductor substrate material and detaching the base substrate from the semiconductor substrate material along with the semiconductor layer at a defective layer. In this case, the quality of the base substrate is inferior to that of the semiconductor substrate material, and the semiconductor substrate material can be repeatedly used for forming other semiconductor substrates after the detachment. Therefore, this method can provide cost reduction and good performance in mass production.
Also, according to the present invention, to achieve the second object, first and second ion-implanted layers may be formed in a semiconductor substrate material at first and second depths different from one another. In this case, first and second base substrates are bonded to both surfaces of the semiconductor substrate material. Then, the first and second substrates are detached from the semiconductor substrate material at the first and second ion-implanted layers, respectively along with the first and second semiconductor layers by a heat treatment. Accordingly, two semiconductor substrates are provided only by one heat-treatment for detachment, resulting in low manufacturing cost. The semiconductor substrate material from which the first and second semiconductor layers are removed can be also reused to manufacture other semiconductor substrates.